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RESEARCH PRODUCT

Amino Acid Chemistry in Solution:  Structural Study and Vibrational Dynamics of Glutamine in Solution. An ab Initio Reaction Field Model

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The self-consistent reaction field (SCRF) theory was used to study structural and vibrational features of the amino acid L-glutamine in solution. Raman and infrared spectra of this molecule in solutions of H 2O and D2O were recorded and measured. The bands were firstly assigned on the basis of the isotopic shifts. An ab initio quadratic force field at the 6-31 +G* level was achieved. The calculation simulated a polar solvent by placing the molecule in an ellipsoidal cavity surrounded by a continuum dielectric. The theoretical results, in terms of structural parameters, vibrational frequencies and descriptions, and infrared intensities, were in satisfactory agreement with the experimental data. The study of the vibrational spectra of amino acids outlines an interesting problem from both theoretical and experimental points of view. The strongly polar topology of these molecules gives rise to intermolecular interactions in condensed phases, which dramatically influences their vibrational dynamics and structural properties. Therefore, in order to investigate the vibrational spectra of an amino acid in solid state or in solution, interactions with the solvent molecules cannot be ignored. In a theoretical treatment of such a problem, solvent molecules can be directly added to the solute molecule, thus allowing for specific solute-solvent interactions. However, when ab initio quantum-chemical approaches are employed, only a few solvent units can be included because of the high computational cost. In addition, nonspecific and long-range interactions are not correctly described. Thus, the polarizable continuum models (PCM) 1,2 have became the most succesful methods to simulate solvent effects. The present work is aimed at the study of the amino acid glutamine in aqueous solution. This is one of the most abundant amino acids in free form, and it occupies a central position in nitrogen metabolism. 3 Prior to this paper, a vibrational study of solid glutamine was reported 4 that was supported on the basis of a normal coordinate calculation from semiempirical methods. 5 Here we report infrared and Raman spectra of this amino acid in solutions of H2O and D2O. The theoretical support was obtained by using a PCM approach in which the solvent effect is simulated by a continuum dielectric. This approach has been adapted to be used with an extensive number of quantum- chemical methods, and we have used that implemented in the GAUSSIAN series of programs. 6 This paper has been organized as follows: first we give the calculation and experimental methods. Then we discuss the obtained results in three sections. Firstly, the molecular structure is discussed and compared to some experimental data. Sec- ondly, we analyze the vibrational spectra of glutamine and its ND3 + ,ND2 deuterated derivative in solution, proposing a tenta- tive assignment for the observed bands. Finally, we present the results of the normal coordinate calculation in terms of frequencies, normal modes, and quadratic force field. In the last section we outline our conclusions.